A bar code field is generally known in the retail trade, for example. Such bar code fields are suitable to mark merchandise of all kinds and contain in coded form numeric information in the form of bar elements of different widths which are typically laid out across the long side of the bar code field. Depending on the application, different types of bar codes are used, e.g., according to MIL-STD-1189 or according to the "European Article Numbering Code".
The support material is most often paper, and the bar elements in the bar code field are therefore applied on a support by means of a simple printing process in a color that contrasts with the background.
Optical readers are known from U.S. Pat. No. 4,743,773 for example. The optical reader scans the bar code field in the longitudinal direction by means of an incident light beam, records the intensity values of the reflected radiation, converts these values into electric signals and transmits these to an evaluating device. The bar code is self-timing, so that the reader and the evaluating device are able to recognize the relative widths of bar elements following each other even when the reading speed is changed constantly. The evaluating device converts these signals into an appropriate code (e.g. binary number) and into an impulse signal by means of a predetermined algorithm for the purpose of further processing.
The French patent 2,490,848 and the U.S. Pat. No. 4,743,744 describe such systems comprising an optical reader and of an evaluating device which are able to read a bar code from the bar code field.
For certain applications the ease with which the bar code field can be produced is of great advantage. On the other hand it would be desirable if the bar codes could be used for the identification of documents such as bank notes for example, because of the simple and reliable readout afforded by bar codes. However, the ease of production rules out the application of conventional bar codes in such cases.
Furthermore, machine-readable diffraction-optical markings which are difficult to copy are known. They are for example embossed in the form of microscopic relief structures into a thin thermoplastic layer applied on paper, are provided with an optically active layer and are protected with a transparent coating material.
The relief structures can have cross-sections of known periodic functions with special frequencies of over 10 lines per millimeter effective for the diffraction of visible light. Limits due to manufacturing restrict the practically useful range to approximately 2,500 lines/mm. But cross-sectional forms with aperiodic functions which contain local spacial frequencies in that range, such as for example matte structures, are also usable. The difference in height of these relief structures is typically selected to lie between 50 nm and 10,000 nm.
Similar relief structures diffract incident light and contain authenticity information, for example in the form of images, the colors and luminosity of which depend on the viewing angle or the movement effects of which depend on the change of the viewing angle.
Such documents and processes for their manufacture are described in Swiss patent 594 936 and in Swiss patent application 00805/88-4.